I've given this (living on a cruise ship while still 'working') more than a passing thought. In the long run, it doesn't fit my current lifestyle, primarily the whole family commitments thing. But if you throw in "if I were single then I could...", there are several 'knowledge worker' jobs that could plausibly be done from literally anywhere in the world. Two such gigs that I could personally undertake:
- Contract Programmer / Web Developer - The bulk of your work is done 'offline' and as long as you have someone else finding the gig for you (IE you become a permanent sub-contractor), there is no need for an always-on internet connection. You could schedule all required calls/meetings to occur while you were in port, where you would also presumably have access to solid internet connectivity through cellular data roaming.
- Contract Information Security 'Penetration Testing'. The bulk of this gig revolves around using automated tools to do the heavy lifting for you; these can be setup within, and scheduled to run automatically from, web services such as AWS. You would only need to download the results from your automated tools and spend a good amount of time (offline) making sense of the data. You could then do the work that requires an internet connection (the actual penetrations and validations) while in-port with cellular internet access. The last (and most time consuming) task is to compile reports on your engagement results, which again can be done completely offline.
I'm sure there have got to be a lot of other similar gigs in other industries. Basically, you need to find a 'knowledge job' that depends on you working independently (self-supervised) most of the time, with maybe 8-12 hours of meetings/calls/web presentations per week, which can be scheduled to occur while you are 'in port'.
To be perfectly honest, I am seriously considering such a move to take place after my kids are out of the house in a few years. That is, assuming, that such gigs still exist in my field at that time!
